Although there is a dedicated watchdog timer in BF, since RTC has a separate logic power, I a feeling that using RTC interrupt may be more reliable than using dedicated watchdog timer.
What is the most reliable method as watchdog functionality for BF531/532/533?
(From HW manual)
RTC Programming Model
The primary Real-Time Clock functionality, shown in Figure 16-1 on page 16-3, consists of registers and counters that are powered by an inde-pendent RTC Vdd supply. This logic is never reset; it comes up in an unknown state when RTC Vdd is first powered on.
This allows the stopwatch o be used as a watchdog timer with a precision of one second. Writing the running stopwatch to 0 forces it to stop and interrupt early.